An optical information storage medium is used for recording information in the form of a series of pits arranged to form a myriad of coaxial, spiral or linear information tracks provided in the record medium. Such an information storage medium is advantageous for implementing an information recording and reproducing disc of the mass storage type for its high storage density as well known in the art.
An information storage medium of this nature is typically provided in the form of a multi-layer structure which consists of a transparent substrate and an information carrying layer of a low-melting point metal deposited on the substrate. As such a low-melting point metal is used, for example, tellurium or bismuth or any alloy containing one or both of these, as disclosed in Japanese Patent Specification No. 54-15483. Similar optical information storage materials are shown in, for example, U.S. Pat. Nos. 3,971,874 and 4,188,214.
From the viewpoint of providing a high degree of optical sensitivity, tin in particular is preferred as a low-melting point metal for use in such an information storage medium. A known information storage medium using tin as a low-melting point metal is however not fully acceptable for achieving information reproducing signals of satisfactory quality and has not been used for practical purposes.
The requirements of an information storage medium for use on an optical information recording and reproducing system include a high degree of reflectivity which is stable at various wavelengths of the incident radiation used. This requirement is of importance for providing stability of operation of the focus and tracking servo systems included in the recording and reproducing system. In order to achieve a desired degree of reflectivity in an optical information storage medium using a low-melting point metal for its information carrying layer, it is required to have the thickness and/or the chemical composition of the layer. This results in a difficulty in providing sustained recording and reproducing capabilities for the resultant information storage medium.
It is, accordingly, an important object of the present invention to provide an improved optical information storage medium which is acceptable for achieving information reproducing signals of satisfactory quality.
It is another important object of the present invention to provide an improved optical information storage medium which is highly sensitive to an incident beam of radiation such as a laser beam and which has high and stable reflectivity providing sustained recording and reproducing capabilities.
Yet it is another important object of the present invention to provide an improved optical information storage medium which is easy and economical to manufacture and which is adapted for use on a practical basis.